Known electrophotographic photoreceptors for use in electrophotography include those utilizing inorganic photoconductive substances, as well as those utilizing organic photoconductive substances (the latter will hereinafter be referred to as "organic electrophotographic photoreceptors"). Organic electrophotographic photoreceptors have been widely used because of advantages in productivity, cost, safety, etc. In recent years, in order to improve their electrophotographic properties, such as charge retention, repeat stability, response to light, spectral characteristics, mechanical strength, etc., there have been proposed various organic electrophotographic photoreceptors of the function separation type wherein functions of an electrophotography photoreceptor are separately borne by a plural elements In such organic electrophotographic photoreceptors of the function separation type, a charge generating layer and a charge transporting layer are formed on an electrically conductive substrate. It is known to use, as a charge generating material for the charge generating layer, such organic compounds as bis-azo pigments (as described in U.S. Pat. No. 4,314,015), condensed polycyclic quinone pigments (as described in JP-A-47-18544) (The term "JP-A" as used herein means an unexamined published Japanese patent application.), and the like.
However, organic electrophotographic photoreceptors utilizing such organic pigments are not satisfactory in sensitivity, spectral characteristics and repeat stability since organic pigments do not possess a flat spectral sensitivity and are capable of generating photo carriers only in low efficiencies.
It has been proposed in JP-A-52-120834 and JP-A-53-27033, in order to improve the above disadvantages, to use as a charge generator an inorganic photoconductive material, such as selenium or alloys of selenium, in particular, trigonal selenium, instead of organic pigments Such inorganic photoconductive materials are highly useful and capable of providing an electrophotographic photoreceptor which is excellent in such electrophotographic properties as photosensitivity, repeatstability, etc. However, conventional function separation type electrophotographic photoreceptors utilizing selenium or an alloy of selenium suffer from the disadvantage that the decay rate of their surface potential becomes lower with a decrease in voltage; namely, their photosensitivity is highly dependent on their surface potential.